In order to develop and design the equipment used in a CNG system to meet the system requirements, generally the arrangement of the pipes and the position of the CNG filling receptacle in the vehicle with the CNG system often are located in the engine compartment. In other words, the locations of the filling receptacle for gasoline and that for CNG are separated apart from one another. As a result, this makes it inconvenient for a motorist to locate the parking spot to have either type of the fuels filled in. In this respect, a concept to relocate the CNG filling receptacle in the engine compartment to be installed together with the gasoline filling opening on the side of the car body is initiated. With such concept, it will make the fuels filling experience more convenient.